The field of the present invention is transceivers using lasers as the communicating medium.
A phenomenon referred to as scintillation causes the random fading of signals transmitted through the atmosphere. It is understood that the atmosphere is not homogeneous in that the index of refraction of air is not constant due to wind or turbulence. The transmission of a beam of light through the atmosphere is subject to these variations in the index of refraction such that the beam may be momentarily deflected from a straight path. With such deflection, an observer of the beam perceives the source to be flickering. Such flickering is highly disruptive to data transmission. A solution may be found in the increase in size of the apertures of the sending and receiving units. The intensity of the source can to a certain extent mitigate losses in transmission where the sensitivity of the receiver is not correspondingly decreased. Often, physical and practical limitations detract from such solutions.
To significantly overcome the effect of scintillation, spacial diversity transmitters have been constructed which employ multiple diode lasers arranged to produce displaced parallel beams. As these beams diverge, they overlap one another. A receiver displaced from the transmitter thus receives uncorrelated light at the receiver when aligned with the beams. As it is unlikely that all beams will be simultaneously diverted, the receiver is able to receive uninterrupted data from at least some of the plurality of transmitters. It has been found that the normalized standard deviation of the intensity at the receiver is reduced by the square root of the number of transmitting elements when properly separated. Reference is made to W. M. Bruno, R. Mangual, & R. F. Zampolin, Diode Laser Spacial Diversity Transmitter, pp. 187–194, SPIE vol. 1044, Optomechanical Design of Laser Transmitters and Receivers (1989), the disclosure of which is incorporated herein by reference.
One structural application of the very principles presented in the foregoing publication are found in U.S. Pat. No. 5,777,768, the disclosure of which is also incorporated herein by reference. Laser transceivers using spaced multiple laser transmitters are used for two-way communication.